1. Field of the Invention
The present invention provides composite coating compositions that include quasicrystalline metal alloys, composite metallic coatings derived from these compositions, and methods of forming these composite coating compositions and composite metallic coatings.
2. Background of the Related Art
A quasicrystal is a phase of solid matter that exhibits long-range orientational order and translational order like a crystal, but whose atoms and clusters repeat in a sequence defined by a sum of periodic functions whose periods are in an irrational ratio. Though expected on the grounds of mathematics for two or three decades, real quasicrystalline metal alloys were discovered only about ten years ago. These still partly mysterious materials have generated a considerable effort to understand their structure and investigate their fundamental properties. The definition of atom positions within a lattice that is incompatible with the translational generative symmetry of conventional crystals has received great attention. It is now best understood in the framework of the so-called high dimensional crystallography. FIG. 1 is an image of a quasicrystalline metal alloy powder at a magnification of 10,000 times.
The question of formation and stability of quasicrystals is of great fundamental importance but yet still obscure. It has been reported that stable icosahedral-type crystals may be grown by a slow solidification technique. Furthermore, the discovery of a supposedly perfectly stable icosahedral phase close to the composition Al64Cu24Fe12 launched a systematic investigation of the Al—Cu-3d metal alloy systems. The Al64Cu24Fe12 alloy was found to grow single crystals with either dodecahedral or icosidodecahedral morphologies. The stable decagonal phase forms in Al—Cu—Co alloys as well as in the vicinity of the composition Al66Cu18Fe8Cr8, growing characteristic needle-shaped deca-prismatic single crystals.
In fact, a careful study, using diffraction techniques, of the Al64Cu24Fe12 single crystals demonstrated that the actual structure is not truly quasicrystalline at room temperature. It is rather that of a crystalline material with a giant unit cell that very closely resembles the quasicrystalline phases. As a matter of fact, the Al64Cu24Fe12 alloy forms a rhombohedral crystal. However, these crystalline, so-called approximant phases, transform irreversibly into the corresponding true quasicrystalline phase when heated up to a temperature range of 650 to 750° C.
Surface mechanical properties of quasicrystalline metallic coatings with three different compositions: Al65Cu20Fe15, Al64Cu18Fe8Cr8 and Al67Cu9Fe10.5Cr10.5Si3 (atomic percent) were examined by Dubois et al. using scratch indentation tests under diamond and hard steel indenters. These metal alloy coatings were prepared by thermal spraying techniques of three types: flame spray, thermal spray, and plasma spray. The structure of the quasicrystalline metal alloys was found to be sensitive to the cooling rate achieved during preparation, e.g., melt spinning or thermal projection technique. In the case of an Al64Cu18Fe8Cr8 alloy, an almost pure decagonal phase or an almost pure icosahedral phase could be obtained, depending on whether a low surface velocity (12 m/s) or high surface velocity (50 m/s), respectively, of the melt spinning wheel was used.
The influence of some coating parameters such as surface roughness, thickness, hardness, and porosity on friction has also been studied. Practically no transfer layer buildup is observed on the contact surfaces of the quasicrystalline metal alloys and indenter materials. A model showing the relation between the coefficient of friction and the roughness parameter has been proposed for a steel ball indenter and is in good agreement with experimental results. A geometrical relation between the depth of a spherical tip and the applied force has also been given. Coefficients of friction of the as-cast alloys as low as 0.09 or 0.13 (measured at constant load of 20 N) were found with diamond or hard steel indenters, respectively, whereas coefficients of friction of 0.07 (with diamond indenter) or 0.19 (with steel ball indenter) were found in the case of coatings. The dynamic hardness was found to vary from 3 to 3.3 GPa for the as-cast alloys and from 1.4 to 2.4 GPa for the coatings.
The hardness of quasicrystalline metal alloys is quite high (H≅9.5 GPa) compared with hardened steel (H=7.7 GPa), and is comparable to that of single-crystalline silicon (H=10.0 GPa). The value of the modulus of elasticity for quasicrystalline metal alloys (E≈140 GPa) is again comparable to that of silicon (E≈168 GPa). The fracture toughness of quasicrystalline metal alloys (Kic=1.0 MPa m1/2) also compares well with that of silicon crystals (Kic=0.7 MPa m1/2).
When scratching with diamond indenters, Al—Cu—Fe—B and Al—Si—Cu—Fe, exhibit low ploughing type friction (respectively μ=0.06±0.005 and 0.07±0.005), very close to values found for Al—Cu—Fe. When scratching with tungsten carbide (WC) indenters it was confirmed that friction is enhanced (respectively μ=0.12±0.03 and 0.10±0.05). Over the first five passes under load of 30 N, the friction remains constant and no scratch brittleness is observed, and friction builds up rather gradually over 300 passes reaching the value of μ≈0.15.
The oxidation of Al—Cu—Fe and Al—Pd—Mn quasicrystalline metal alloys is very similar. Both alloys have effective protection from rapid oxidation up to temperatures of 750° C. However, comparison of the Al—Cu—Fe quasicrystalline material to Al—Pd—Mn shows that the latter one is more readily oxidized. Further, above temperatures of 750° C. enrichment of Al on the surface takes place leading to slight changes in the quasicrystalline structure.
Using three analyses of the wetting phenomenon (thermodynamics, electronic, and hysteretic), it is suggested that quasicrystalline metal alloy coatings should exhibit non-stick properties. Further, it has been shown that quasicrystalline metal alloy coatings have low surface tension, and pin liquid efficiently. This low surface tension property has another important physical consequence of providing non-oxidizability.
Many quasicrystalline metal alloys are obtained by rapid solidification of a liquid melt. The procedure is similar to the production of metallic glasses, where cooling rates of 105 to 109 Kelvin per second (K/s) are necessary to avoid the nucleation of high-temperature equilibrium phases. Melt spinning is one of the techniques that permit supercooling variation of rate at the nucleation state. Here, molten alloys are squirted on to a rotating wheel, liquid is quenched at a rate of 106 K/s and the sample is obtained as ribbons a few micrometers (μm) thick and a few millimeters (mm) wide. The ribbons contain single grains of quasicrystalline material with sizes of about 1 μm across suitable only for electron diffraction characterization. Unfortunately, formation parameters are difficult to control and though single-phase quasicrystals can be produced in this manner, their reproducibility is poor.
All current methods for the production of quasicrystals (as well as of metastable alloys and glasses) are based on generating disorder at the atomic level. This is generally done by a solid-state reaction. A typical method is the multilayer deposition technique in which alternating layers of e.g. Al and Mn are deposited on a substrate, the thickness being of the order of 1000 Å. Once the multilayer with the right average composition is obtained, the sample is bombarded by high-energy ions of inert gases (e.g., Xe2+). An amorphous, quasicrystalline or crystalline state is obtained, depending on the energy of the ions and the sample temperature. Here, disorder is introduced by the kinetic energy of the ions and is also driven by the temperature of the multilayer sample, since atoms become more mobile as the temperature increases. Single quasicrystalline phases can be obtained in this manner, but the samples are quite small (2×2 mm2 and 1000 Å thick).
Mechanical alloying is another method to produce amorphous, quasicrystalline or crystalline states. Powders of different elements are alloyed by the kinetic energy of balls vibrating in a steel container. Two other techniques used are the evaporation technique and the laser or electron melting of thin layers. In the former method, a fog of small droplets of liquid alloy is produced, and quenched. Various external shapes and structures are obtained, with typical sizes in the range of 500 to 3000 Å. Conventional casting (i.e., slow cooling from the melt) has also been employed to obtain a stable quasicrystalline state, at least in some composition and temperature range.
Despite the interesting set of physical properties exhibited by quasicrystalline metal alloys, these materials have not found their way into many commercial applications due in large part to the difficulty and expense of forming quasicrystalline metal alloy components or coatings. While plasma sprayed quasicrystalline metal alloy coatings have been used, with limited success, to form a non-stick surface on cookware, the commercial production of this cookware has ceased. FIG. 2 is an SEM image of a quasicrystalline metal alloy coating after plasma deposition onto a substrate. As shown, the severe process conditions of the plasma spray have altered the form of the quasicrystals and formed a non-uniform coating.
Therefore, there is a need for a method of forming a coating composition that exhibits the same physical properties, such as high wear resistance and low friction, as a single quasicrystalline metal alloy material. It would be desirable if the coating composition could be formed on a substrate as a metallic coating. It would be even more desirable if the method could form uniform and adherent coatings having various desired thicknesses. Still further, it would be beneficial if the coating composition and metallic coating derived from it could be formed economically under processing conditions that are compatible with the use of various substrates and applications.